TDA7406T datasheet(31/49 Pages) STMICROELECTRONICS

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TDA7406T

4.9 Highcut Control

The highcut control set-up is similar to the stereoblend control set-up: the starting point VHCH can be set with

2 bits to be 42, 50, 58 or 66% of REF5V whereas the range can be set to be 17, 22, 28 or 33% of VHCH (see

fig. 26).

Figure 26. Highcut characteristics

FUNCTIONAL DESCRIPTION OF THE NOISEBLANKER

In the automotive environment the MPX-signal as well as the AM-signal is disturbed by spikes produced by the

ignition and other radiating sources like the wiper-motor. The aim of the noiseblanker part is to cancel the audi-

ble influence of the spikes. Therefore the output of the stereodecoder is held at the actual voltage for a time

between 22 and 38µs in FM (370 and 645µs in AM-mode). The block diagram of the noiseblanker is given in

fig.27.

Figure 27. Block diagram of the noiseblanker

In a first stage the spikes must be detected but to avoid a wrong triggering on high frequency (white) noise a

complex trigger control is implemented. Behind the trigger stage a pulse former generates the “blanking”-pulse.

5.1 Trigger Path FM

The incoming MPX signal is highpass-filtered, amplified and rectified. This second order highpass-filter has a

corner-frequency of 140kHz. The rectified signal, RECT, is integrated (lowpass filtered) to generate a signal

called PEAK. The DC-charge/discharge behavior can be adjusted as well as the transient behavior (MP-dis-

charge control). Also noise with a frequency 140kHz increases the PEAK voltage. The PEAK voltage is fed to

a threshold generator, which adds to the PEAK-voltage a DC-dependent threshold VTH. Both signals, RECT

and PEAK+VTH are fed to a comparator which triggers a re-triggerable flip-flop. The flip-flop's output activates

the sample-and-hold circuits in the signal path for the selected duration.

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